1. Field of the Invention
This invention relates to a process for the purification of dimethylphenol. More particularly, it relates to a process for separating high-purity 3,4- or 3,5-dimethylphenol from a dimethylphenol-containing fraction by distillation and melt crystallization.
2. Description of the Prior Art
Dimethylphenol, which is also called xylenol or hydroxyxylene, is used as a starting material in the production of phenolic resins, medicines, pesticides, and the like. It is present in tar acids derived from coal tar fractions and has six isomers: 2,3-, 2,4-, 2,5-, 2,6-, 3,4-, and 3,5-isomers.
Many products are derived from coal tar by the following procedures. Coal tar is subjected to primary distillation to separate into several fractions, such as light oil, carbolic oil, naphthalene oil, wash oil (absorbing oil), and anthracene oil, leaving a soft pitch having a low softening point as a residue. Each fraction may be further subjected to appropriate separation or purification procedures such as distillation, extraction, and crystallization, as required, to recover the desired products.
Tar acids are obtained from various coal tar fractions such as carbolic oil, naphthalene oil, and wash oil by extracting these fractions with an aqueous sodium hydroxide solution and then neutralizing the resulting extract, which contains a tar acid as its sodium salt. Tar acids are mixtures of various phenolic compounds and are usually separated by fractional distillation into several fractions, e.g., phenol, o-methylphenol, m- and p-methylphenol, and dimethylphenol fractions.
The dimethylphenol fraction of a tar acid contains all or some of the six isomers of dimethylphenol as well as ethylphenols and trimethylphenols. The melting and boiling points of these compounds are shown in Table 1 below.
TABLE 1 ______________________________________ Compound (Abbrev.) M.p. (.degree.C.) B.p. (.degree.C.) ______________________________________ 2,3-Dimethylphenol (2,3-DMP) 72.6 216.9 2,4-Dimethylphenol (2,4-DMP) 24.5 210.9 2,5-Dimethylphenol (2,5-DMP) 74.9 211.1 2,6-Dimethylphenol (2,6-DMP) 45.6 201.1 3,4-Dimethylphenol (3,4-DMP) 65.1 226.9 3,5-Dimethylphenol (3,5-DMP) 63.3 221.7 2-Ethylphenol (2-EP) &lt;-18 207.7 3-Ethylphenol (3-EP) -4.0 214.0 4-Ethylphenol (4-EP) 44.8 219.0 ______________________________________
In order to use a compound in the dimethylphenol fraction as a starting material in a chemical synthesis, it is necessary to separate the compound from the fraction. However, it is very difficult to separate 3,4-dimethylphenol and 3,5-dimethylphenol individually since both the boiling points and melting points of these two isomers are close to each other, as shown in Table 1 above. Therefore, it is impossible to recover 3,4- or 3,5-dimethylphenol of high purity by rectification even if a special rectifying column with a great number of plates or a high reflux ratio is employed.
Commercial production of 3,4- and 3,5-dimethyphenols has had to rely on complicated chemical synthesis, and as a result, these substances are relatively expensive. Accordingly, there is a need for a process capable of separating 3,4- or 3,5-dimethylphenol of high purity from a dimethylphenol fraction of a tar acid or a similar source which contains both 3,4- and 3,5-dimethylphenols so as to obtain 3,4- or 3,5-dimethylphenol inexpensively and in large quantities.
Crystallization is also a well-known separation technique and is often employed when separation by distillation is difficult or inconvenient.
The present inventors attempted to separate 3,4- or 3,5-dimethylphenol by means of melt crystallization. Specifically, a dimethylphenol fraction obtained by distillation of a tar acid was rectified to recover a 3,4- or 3,5-dimethylphenol-rich fraction, which was then cooled with agitation to precipitate crystals of the desired 3,4- or 3,5-dimethylphenol, and the resulting slurry was centrifuged to separate the crystals from its mother liquor. However, it was found that this procedure could not provide 3,4- or 3,5-dimethylphenol crystals of high purity.
In Japanese Patent Application Kokai No. 63-222137(1988) the present inventors proposed that a dimethylphenol isomer of high purity can be separated from a dimethylphenol fraction by distilling the fraction so as to give a distillate which contains at least 35% by weight of the desired isomer to be separated and in which the concentration of each of the other dimethyphenol isomers is not greater than a prescribed value defined by inequality (I) given below, and then subjecting the distillate to crystallization from a melt, i.e., melt crystallization.
However, when the above-mentioned method is used to separate 3,4- or 3,5-dimethylphenol from a dimethylphenol fraction, the desired 3,4- or 3,5-isomer of high purity cannot be obtained from the distillate by melt crystallization and subsequent centrifugal separation unless the distillate contains at least 70% by weight of 3,4- or 3,5-isomer. This is because it is very difficult to completely remove the mother liquor from the crystals by means of centrifugal separation. In order to obtain the desired isomer of high purity, it is therefore necessary to wash the separated crystals with an organic solvent, such as hexane or heptane, which can readily dissolve the mother liquor. The use of an organic solvent, however, is not desirable from an ecological viewpoint and is disadvantageous in that it involves the steps of removing the solvent from the crystals and recovering the removed solvent, thereby making both the process and apparatus complicated. Furthermore, the yield of the desired isomer is greatly decreased by washing.
Other purification methods for dimethylphenols are disclosed in Japanese Patent Application Kokai Nos. 49-117435 (1974), 57-165333(1982), 58-52235(1983), 60-136528(1985), 60-248638(1985) and 63-303938(1988), as well as Japanese Patent Publication Nos. 52-38545(1977), 58-13527(1983), 58-58330 (1983), 61-31088(1986), 62-1933(1987), 63-6533(1988) and 63-6534(1988).